In Situ Studies of the Formation of MoP Catalysts and Their Structure under Reaction Conditions for Higher Alcohol Synthesis: The Role of Promoters and Mesoporous Supports

Abstract

Three formulations of a molybdenum phosphide (MoP) catalyst system were characterized for the higher alcohol synthesis (HAS) reaction using in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), monitoring the chemical phase evolution during activation and under reaction conditions. The in situ study herein provides important insights into the effect of the support and of K that lead to high performance in HAS for K-promoted MoP supported on carbon, as evidenced by previous studies (ethanol selectivity: 29%; conversion: 5%). During the activation process, XAS shows that the carbon-supported samples reduce and reach a highly crystalline state at a lower temperature than the SBA-15-supported sample, indicating a substantial difference in catalyst activation. After activation, the samples are introduced to relevant reaction conditions resulting in spectra fairly similar to one another. XRD results corroborate the difference in the degree of crystallinity of these samples, in alignment with the XAS, and reveal the formation of crystalline potassium pyrophosphate (K 4 P 2 O 7 ) during the activation period of the K-promoted samples. This K 4 P 2 O 7 phase remains present under reaction conditions. Taken together, these results provide insights into the roles played by the carbon support and K promotion, connecting activity to electronic and crystal structure.

One-Step Synthesis and Operando Electrochemical Impedance Spectroscopic Characterization of Heterostructured MoP–Mo2N Electrocatalysts for Stable Hydrogen Evolution Reaction

“…For the MoP electrode, it is apparent that MoP forms on the electrode because of obvious characteristic peaks at 27.8°, 31.9°, 42.8°, and 67.2°, being indexed to the (001), (010), (011), and (012) lattice planes of MoP based on ICSD#98−007−6367. 48 The characteristic diffraction peaks related to Mo are also evident because they are located at 40.5°, 58.6°, and 73.7°, corresponding well to the ( 011 49 Therefore, we can deduce that the phospho-nitriding process on the Mo mesh can create the Mott−Schottky MoP−Mo 2 N@ Mo heterostructured electrocatalysts. Thus, the XRD data validate and correlate with the proposed mechanism (schematic representation; Figure 1) for the formation of MoP−Mo 2 N@Mo compounds upon one-step treatment.…”

“…The XRD patterns recorded for the MoP and MoPN after phosphidation and phospho-nitriding processes are shown in Figure a. For the MoP electrode, it is apparent that MoP forms on the electrode because of obvious characteristic peaks at 27.8°, 31.9°, 42.8°, and 67.2°, being indexed to the (001), (010), (011), and (012) lattice planes of MoP based on ICSD#98–007–6367 …”

Bridging the Gap between the X-ray Absorption Spectroscopy and the Computational Catalysis Communities in Heterogeneous Catalysis: A Perspective on the Current and Future Research Directions

“…In conjunction with other characterization techniques and kinetic measurements, XAS has played a prominent role in developing structure–property relationships and elucidating mechanistic pathways for various catalytic reactions. − Indeed, while X-ray absorption near-edge structure (XANES) spectroscopy provides information about the electronic structure (e.g., the average oxidation state of a specific element in the experimental sample), extended X-ray absorption fine structure (EXAFS) spectroscopy probes the local bonding environment around the absorbing atom. Coupled with the easy ability to tune X-ray energies from ∼2 to ∼30 keV (covering the absorption edges of virtually all catalytically active elements) at the synchrotron, XAS is a versatile technique that is used to study a vast range of catalytic materials, including industrially relevant catalysts.…”

Bridging the Gap between the X-ray Absorption Spectroscopy and the Computational Catalysis Communities in Heterogeneous Catalysis: A Perspective on the Current and Future Research Directions

“…5,[23][24][25][26] In conjunction with other characterization techniques and kinetic measurements, XAS has played a prominent role in developing structure-property relationships and elucidating mechanistic pathways for various catalytic reactions. [27][28][29][30][31] Indeed, while X-ray absorption near edge structure (XANES) spectroscopy provides information about the electronic structure (e.g., the average oxidation state of a specific element in the experimental sample), extended X-ray absorption fine structure (EXAFS) spectroscopy probes the local bonding environment around the absorbing atom.…”